Foster, who was forced off during the goalless draw with Everton on Saturday, will undergo surgery on his foot in the next few days.
The news will come as blow to England manager Roy Hodgson who was set to name Foster in his squad for the World Cup qualifiers against Moldova and Ukraine.
Albion manager Steve Clarke will now pass the goalkeeping duties on to Wales international Boaz Myhill, who returned to training on Monday after a wrist injury, or Luke Daniels, who made his first-team debut at Everton by coming on for Foster.
Clarke told the club's official website: "There's no hiding the fact this is disappointing news as Ben is a key player for us. He was excellent on Saturday before his injury forced him off.
"In the meantime, I've got two other very good goalkeepers to choose from.
"Boaz did well when he deputised for Ben last season and Luke showed on Saturday he is a more than capable goalkeeper."